Silicon wafers or sheets may be used in, for example, the integrated circuit or solar cell industry. Demand for solar cells continues to increase as the demand for renewable energy sources increases. One major cost in the solar cell industry is the wafer or sheet used to make solar cells. Reductions in cost to the wafers or sheets may reduce the cost of solar cells and make this renewable energy technology more prevalent. One promising method that has been investigated to lower the cost of materials for solar cells is the horizontal ribbon growth (HRG) technique where crystalline sheets are pulled horizontally along the surface of a melt. In this method, a portion of a melt surface is cooled sufficiently to locally initiate crystallization with the aid of a seed, which may be then drawn along the melt surface to form a crystalline sheet. The local cooling may be accomplished by providing a device that rapidly removes heat above the region of the melt surface where crystallization is initiated. Under proper conditions a stable leading edge of the crystalline sheet may be established in this region.
In order to ensure growth stability, it may be useful to provide sufficient heat flow through the melt directly under the leading edge of the crystalline sheet. It may also be desired to reduce the thickness of the crystalline sheet such as below 200 um, which also requires uniform heat flow under the already-formed portions of the crystalline sheet. However, achieving controlled heat flow within a silicon melt is very challenging for several reasons. Firstly, molten Si has a very high thermal conductivity, so that any heat introduced at the bottom of a crucible that contains the melt spreads out before reaching the melt surface. In addition, the crucible material used to contain silicon melt is fused silica, due to its resistance to reaction with silicon at elevated temperature. However, fused silica is a good thermal insulator such that a large thermal gradient is required to conduct substantial heat to the silicon melt. In turn, this requires that the outer temperature of the fused silica crucible being heated is maintained at a much higher temperature than the melt temperature. However, fused silica softens to an unacceptable degree above 1880 K, which limits the amount of heat flow that can be introduced into the melt. Accordingly present day apparatus may not provide sufficient heat flow to ensure stable growth of a crystalline sheet.
It is with respect to these and other considerations that the present improvements have been needed.